1. Field of the Invention
This invention relates to a technology for measuring a propagation delay time of a cable assembly with a high precision.
2. Description of the Related Art
In a notebook personal computer and the like, a down-sizing and a weight reduction as well as an increase in speed and resolution are currently in progress, which requires a cable assembly having a high-speed transmission capability. With increasing speed of signal transmission in a cable, a range of skew, which is a difference in a propagation delay time that is allowed for the cable assembly, is getting narrower. This is because when a difference in the propagation delay time occurs, the synchronization between signals tends to shift and errors of signals tend to occur. In the case of high-speed transmission, a particularly small difference in the propagation delay time is required. For this reason, it is necessary to measure the difference of the propagation delay time to check if the difference of the propagation delay time is within a predetermined range.
FIG. 20 is a schematic for explaining the measurement of a propagation delay time of a cable assembly 30 using a conventional measuring apparatus 110. The measuring apparatus 110 includes a conversion adapter 104, to which a proximal end 30a of the cable assembly 30 is connected directly. The distal end 30b of the cable assembly 30 is electrically open. In this connection state, the measuring apparatus 110 transmits a signal having a predetermined frequency to the cable assembly 30, reflection of the signal occurs at a position where impedance mismatches to generate a reflected wave. In the measuring apparatus 110, time-dependent variation of amplitude of reflection coefficient ρ as a ratio of reflected wave to input wave is obtained, from which a propagation delay time Td is obtained, as shown in FIG. 21.
The time-dependent variation of the amplitude of reflection coefficient ρ obtained in the manner explained in FIG. 21, the amplitude of reflection coefficient ρ increases rapidly with time first at a point PP1 that corresponds to near the proximal end 30a of the cable assembly and then at a point PP2 that corresponds to the proximal end of the cable assembly 30, which proximal end is an open end. The time-dependent variation of the amplitude of reflection coefficient ρ is output to a displaying unit 107 of the measuring apparatus 110. The measurer visually measures a rapidly changing difference in time between the two points PP1 and PP2 and a half of the difference in time is defined as a propagation delay time. This measurement method is called an open method since the distal end 30b of the cable assembly is open.
On the other hand, a short-circuit method with which the distal end 30b of the cable assembly is short-circuited and then a propagation delay time is obtained. According to the short-circuit method, the distal end 30a of the cable assembly 30 is short-circuited with a short-circuiting pin 130 before the propagation delay time can be obtained. FIG. 23 is a schematic for explaining time-dependent variation of the amplitude of reflection coefficient obtained by the conventional short-circuit method. The amplitude of reflection coefficient ρ rapidly decreases at the point PP2 that corresponds to the distal end 30a of the cable assembly 30, which is different from the time-dependent variation of the amplitude of reflection coefficient ρ obtained by the open method. However, also in the short-circuit method, a half of a difference in time between the points PP1 and PP2 at which the amplitude of reflection coefficient ρ rapidly changes is defined as a propagation delay time.
When either the open method or short-circuit method is used, near the points PP1 and PP2, the amplitude of reflection coefficient varies drastically billows with high amplitudes. Accordingly, the measurer may wrongly read the position of the point PP1, which makes it difficult to perform the measurement of propagation delay times with a high precision. For example, the position of the point PP1 is read to be Pt1 or Pt2.
This means that the error by the measurer increases and the propagation delay time is measured with a low precision.